make batchConn more generic

[zjx20]

Sometimes we would like to implement a custom net.PacketConn, for example the tcpraw. This patch allows kcp-go to leverage the batching ability provided by the custom conn, if possible.

[codecov]

Codecov Report

Merging #200 (b930b19) into master (7450e91) will increase coverage by 0.82%.
The diff coverage is 86.17%.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   82.47%   83.29%   +0.82%     
==========================================
  Files          12       11       -1     
  Lines        1889     1910      +21     
==========================================
+ Hits         1558     1591      +33     
+ Misses        260      250      -10     
+ Partials       71       69       -2     

Impacted Files	Coverage Δ
batchconn_linux.go	72.22% <72.22%> (ø)
readloop.go	89.28% <92.10%> (+5.95%)	⬆️
sess.go	84.06% <100.00%> (+0.59%)	⬆️
tx.go	100.00% <100.00%> (+30.00%)	⬆️

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